1. Field of the Invention
This disclosure relates generally to methods and compositions to tailor polymers for use in the oil field services industry. More specifically, this invention relates to methods and compositions to tailor polymers to modify the viscosity of oil field services fluids.
2. Description of the Related Art
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Guar and guar derivatives are widely used in fluids for use in oil-well fracturing and stimulation applications. They are particularly used in hydraulic fracturing fluids, to initiate and propagate the hydraulic fracture, to provide rheology to transport proppant through the hydraulic fracture, to provide fluid loss control, and to suspend proppant in the hydraulic fractures after a hydraulic fracturing treatment until the hydraulic fracture has closed onto the proppant to hold it in place. They are used in combination with several other chemicals, and particularly crosslinkers to provide optimum crosslinked gels, necessary to transport and suspend the proppant.
The guar used in the oil field services industry can be either natural guar or derivatized guar. The derivatized guar can be hydroxypropyl guar (HPG), cationic guar, carboxymethyl guar (CMG), carboxymethyl hydroxypropyl guar (CMHPG), hydroxyethyl guar (HEG), carboxymethyl hydroxyethyl guar (CMHEG), hydrophobically-modified guar (HMG), hydrophobically-modified carboxymethyl guar (HMCMG) and hydrophobically-modified hydroxyethyl guar (HMHEG).
Conventional natural and derivatized guars have primary and secondary hydroxyl (—OH) groups which are the typical functionality responsible for their crosslinking. In the oilfield, inorganic crosslinkers such as borates, zirconates, titanates, aluminates, chromates, and hafnium are used to increase the gel viscosity. The type of inorganic crosslinker and the conditions used depends on fluid requirements. Borate crosslinkers are commonly used for low and medium temperature applications due to their shear insensitivity. It is commonly accepted that borate ions are responsible for the crosslinking of guar derivatives through interaction with the cis hydroxyls in the positions 2 and 3 of the manose and galactose monosaccharides. Organometalic crosslinkers are also used to increase the high temperature stability of the fluids. Complexes of transition metals such as Zr, Ti, Al, Cr, Hf, are commonly used as metallic crosslinkers. It is commonly accepted that organometallic ions are responsible for the crosslinking of guar derivatives through interaction with either the cis hydroxyls in the positions 2 and 3 of the mannose and galactose monosaccharides, or alternatively with the carboxylate groups introduced in the structure through derivatization of the natural guar polysaccharide. Organometallic crosslinked fluids are typically delayed fluids for which suitable ligands and delay agents are required to achieve acceptable crosslinking delay. Organometallic crosslinkers are typically more effective in crosslinking guar derivatives than natural guar.
In the applications disclosed in the oilfield literature using guar and guar derivatives, it is commonly accepted that all the chemical functionality required for the polymer to effectively crosslink is available from the source polymer (be that functionality hydroxyls in natural guar or hydroxyl and or carboxylate groups in derivatized guar) prior to being pumped into the wellbore.
A typical process for using fluids comprising these polymers includes the steps of hydrating the natural guar or guar derivative in an aqueous medium, providing a crosslinker, and preferably some means of delaying the interaction between polymer or crosslinker. Typically, the use of encapsulated crosslinkers, the delayed release of activators, or the addition of a substantial amount of competing ligands as complexing agents for the crosslinker metal have proven as effective methods in a variety of applications.
The absence of alternative functional chemical moieties to the hydroxyl and or carboxylate groups present in guar and guar derivatives impairs the ability to react these polymers in aqueous medium through other common chemistry reactions. This in turn prevents the use of a variety of other organic crosslinkers in use in other industries.
Abad et al. GB2422839B, disclosed methods to functionalize and further crosslink guar and guar derivatives by introducing alternative functionalities such as epoxy or aldehyde, in the guar backbone.
The use and crosslinking of oxidized guar, aldehyde containing guar, and similar aldehyde or carbonyl containing polymers through chemical reaction of the aldehyde group has been disclosed in the past. Germino et al. U.S. Pat. No. 3,297,604, disclosed the use of galactose oxidase enzyme to produce oxidized guar gum, yielding aldehyde bearing oxidized products which were crosslinked with amino polymers, polyhydroxy containing polymers and proteins. Brady et al., U.S. Pat. No. 6,022,717 described a novel process to oxidize guar using a galactose oxidase enzyme. Segura, GB2416792A discloses a method of treating a subterranean formation with a treating fluid comprising a carbonyl containing compound and an amine containing compound. Such carbonyl compound can be obtained by oxidation of a guar polymer with periodate. Abad et al. GB2422839B also disclose the use of aldehyde containing polymers such as polyacroleine, oxidized guar, oxidized starch, acroleine grafted guar, guar polyaldehyde, and polymers containing aldehyde precursors such as acetals and hemiacetals as viscosifying agents for wellbore operations. Melbouci et al. US2007/0275862 disclose oilfield servicing compositions including fracturing and stimulation fluids containing and aldehyde guar produced by enzymatic oxidation of guar or guar derivative with galactose oxidase combined with catalase and peroxidase.
All the crosslinked treatments described in the prior art only consider the use of functional polymers that have been synthesized away from the wellsite and then transported to the wellsite for use in wellbore treatments. In addition the treatment disclosed in the prior art do not propose methods of delaying the chemical interaction between the functional polymer and the organic crosslinker, and rely on the reaction kinetics and diffusion of the reactive species for suitable crosslinking control. This time delay is critical for some downhole applications such as fracturing, where a non-delayed fluid can cause excessive friction in the pipe, and ultimately inability to pump at the required rate, lack of sufficient fracture width, or undesired pressure increase up to maximum allowed for the treatment, all which can result in failed operations or service quality issues. Time delay is also important in applications where a plugging mechanism is required such as internal filtercake formation, water control, diversion in wellbores for different stages in a treatment, diversion in natural fractures for directional steering of the main hydraulic fracture wing, or diversion in formations of different permeability, loss circulation while drilling, and the like. In these applications substantial viscosity development or gellation is required once the treatment has substantially penetrated the zone of the wellbore or the geologic formation of interest so that appropriate performance is ensured.
In the vast majority of the wellbore applications where viscosification, crosslinking or gellation is required downhole, delaying mechanisms are typically needed. Methods of introducing additional delay to the interaction between functionalized polymers and the organic crosslinkers are needed. Methods to tailor crosslinking systems to modify the viscosity of polymer-based systems are needed. It is desirable that these methods are effective, efficient, and reliable over a wide range of temperatures and pressures.